Diamines and polyamines from the diphenylmethane series (MDA) are understood to be amines and mixtures of amines of the following type:
wherein n stands for a natural number ≧2.
The term monomeric MDA (MMDA) for compounds and mixtures of compounds with n=2 and the term polymeric MDA (PMDA) for compounds and mixtures of compounds with n>2 are also conventional. For the sake of simplicity, mixtures of compounds in which compounds with n=2 and n>2 occur side by side are conventionally grouped together under the term MDA (diamines and polyamines from the diphenylmethane series).
The continuous, discontinuous or semi-continuous preparation of diamines and polyamines from the diphenylmethane series is described in numerous publications and patents (for example H. J. Twitchett, Chem. Soc. Rev. 3 (2), 209 (1974); M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd Ed., New York, 2, 338-348 (1978); EP-A-31 423; EP-B-1 167 343; EP-A-1 403 242; EP 934 922 B1).
In the processes that are employed in industry, MDA is conventionally prepared by reacting aniline and formaldehyde in the presence of acid catalysts, the acid catalyst conventionally being neutralized at the end of the process by addition of a base, the reaction mixture being separated into an organic and an aqueous phase and the organic phase being sent to the final processing stages, such as for example removal of excess aniline by distillation (U.S. Pat. No. 5,310,769; DE-A-198 04 918; JP-A-2004026753).
Common to all of the processes described in the literature for preparing MDA by reacting aniline and formaldehyde in the presence of an acid catalyst is the fact that chromophores are formed during the reaction which discolor the MDA that is produced. During neutralization of the acid catalyst and removal of the aniline that is used in excess in the reaction, these discolorations are not or not adequately reduced or removed, and during the subsequent phosgenation of the MDA to form the corresponding diisocyanates and polyisocyanates and their subsequent processing (separation of the solvent, separation of monomeric MDI), they commonly lead to darkly colored products which in turn result in polyurethane foams having a yellowish discoloration or to other discolored polyurethane (PU) materials. Although the natural color of the diisocyanates and polyisocyanates has no negative influence on the mechanical properties of the polyurethanes that are produced therefrom, light-colored products are preferred because of their good variability in the manufacturer's production process, for example in terms of visibility through thin top coats and colored design possibilities.
There has been no shortage of attempts to reduce the discoloration of MDA.
EP 1 270 544 B1 describes a process for preparing MDA with minimization of the content of undesirable by-products by reacting aniline with formaldehyde in the presence of acid catalysts, characterized in that in a semi-continuous process aniline and optionally acid catalyst are prepared, formaldehyde and optionally acid catalyst are introduced by means of a mixer into a circuit in which aniline, optionally acid and optionally already added formaldehyde are circulated and after introduction of at least 50% of the total amount of formaldehyde to be introduced the reaction mixture is heated to a temperature of greater than 75° C. In particular, minimization of the content of N-methyl MDA is claimed, the reduction of which in MDA according to the teaching of EP 1 270 544 B1 is supposed to lead to a lighter color in a subsequent phosgenation stage to produce a crude MDI.
Improving the color values by reducing the content of N-methyl MDA in the MDA produced, by means of a special introduction of formaldehyde, is also the basis of the following processes.
DD-A-295 628 describes for a discontinuous process adding the formaldehyde in two steps during the condensation stage, the bulk of the formaldehyde being added in the first step at a relatively low temperature and the remaining formaldehyde being added in the second step at the same or a higher temperature.
EP-A-451 442 and DD-A-238 042 disclose for a continuous process adding the formaldehyde in several process steps.
To improve the color values, in addition to minimizing the content of N-methyl MDA, U.S. Pat. No. 5,286,760 also teaches the minimization of the by-products acridan and acridine. U.S. Pat. No. 5,286,760 does not modify the introduction of formaldehyde, however, but rather the rearrangements following the primary reaction of the formaldehyde with aniline. For a continuous MDA production U.S. Pat. No. 5,286,760 describes a partial neutralization of the reaction mixture between the condensation stage of two molecules of aniline and one molecule of formaldehyde and the subsequent rearrangement of the aminobenzylamines formed as intermediates (ABA for short) to form MDA.
U.S. Pat. No. 5,310,769 too intervenes above all in the rearrangements. U.S. Pat. No. 5,310,769 describes a process for preparing polyamines from the diphenylmethane series by condensation of aniline with formaldehyde, subsequent reaction in the presence of an acid catalyst, neutralization of the acid catalyst on completion of the reaction and purification of the resulting mixture of diamines and polyamines by removing the excess aromatic amine by distillation, characterized in that in a preferred variant
a) aniline is reacted with formaldehyde in a molar ratio of 1.5:1 to 10:1 at temperatures of between 10 and 150° C.,
b) after which an acid catalyst is added to the reaction mixture in a molar ratio of aniline to acid catalyst of 2:1 to 100:1 at temperatures of between 10 and 150° C., the water forming in the condensation reaction being separated off either before or after step b),
c) then the temperature of the mixture obtained in step b) is increased by at least 40° C. within 15 minutes and it is then heated further to the final temperature between 105 and 200° C. and the temperature held for 10 to 300 minutes following the increase in temperature.
U.S. Pat. No. 5,310,769 teaches that through the special temperature control during the condensation and rearrangement stages a mixture of diamines and polyamines from the diphenylmethane series is obtained whose subsequent phosgenation makes it possible to obtain particularly light-colored polyurethane foams.
Mixtures of diamines and polyamines from the diphenylmethane series whose subsequent phosgenation leads to polyisocyanates having a strongly reduced coloration are also obtained according to U.S. Pat. No. 4,792,624 by the use of a process which is characterized in that
a) fast-flowing streams of aqueous aniline hydrochloride and aqueous formaldehyde in a ratio of 1.6 to 8 mol of aniline per mol of formaldehyde are mixed together intensively at the entrance to a tubular type reactor, as a result of which a mixture containing aminobenzylamines is immediately formed,
b) the mixture produced according to a) is then passed through a cooled reaction zone in which the content of aminobenzylamines in the mixture increases to at least 30 wt. %,
c) the reaction mixture is removed from the cooled reaction zone at the same rate as reaction mixture flows in from step a),
d) the reaction mixture from the cooled reaction zone is then passed through a rearrangement zone with temperatures of 60° to 200° C., in which the polyamine from the diphenylmethane series is formed,
e) the reaction mixture is removed from the rearrangement zone at the same rate as reaction mixture is fed into the rearrangement zone,
f) the reaction mixture from the rearrangement zone is supplied continuously to a neutralization zone in which the acid components are neutralized, then aniline and water are separated off from the reaction mixture, such that an aniline-free polyamine from the diphenylmethane series is obtained,
g) the polyamine mixture is removed from step f) at the same rate as reaction mixture is introduced into the neutralization or distillation stage, and
h) the bulk of the polyamine mixture obtained is discharged into a storage tank but a partial stream of the polyamine mixture in an amount of 1 to 40 wt. % based on the combined initial weights of the amounts of aniline, aniline hydrochloride and formaldehyde introduced in step a) is returned to step b) and during the ongoing process a) to h) is again passed through steps b) to h).
According to U.S. Pat. No. 4,792,624, to achieve a maximum improvement in color values it is substantial for the excess aniline used in the process to be separated from the MDA before the MDA is added to the benzylamines. Furthermore, according to the teaching of U.S. Pat. No. 4,792,624 the improved coloration is only achieved if the recycled polyamine is added where aminobenzylamines are present and not at the stage in which the aniline and formaldehyde first react.
Common to all of the processes cited and described in the literature for preparing diamines and polyamines from the diphenylmethane series by reacting aniline with formalin in the presence of acid catalysts is the fact that they achieve improvements in the coloration of the mixtures of diamines and polyamines that are formed or of the isocyanates and polyurethanes produced from them by modifying individual process parameters, e.g. the addition of components, the concentration of acid catalyst, the temperature control or the product composition during the rearrangements. There is nevertheless still a need for new processes with still greater color improvements.